1. Field of the Invention
The present invention relates to a surface acoustic wave element used in, for example, a filter for selecting the frequency used in a communication device, or a resonator for an oscillator with high stability.
2. Description of the Related Art
A conventional substrate for a surface acoustic wave element, is typically produced by cutting and polishing a single piezoelectric crystal of lithium niobate, lithium tantalate, lithium tetraborate (see, for example, Japanese Unexamined Patent Publication No. JP-A-60-41315), crystallized quartz, or the like, in accordance with a particular cut face.
Important properties of a piezoelectric substrate used in a surface acoustic wave elements include the temperature coefficient of delay (TCD) and the electromechanical coupling coefficient (K.sup.2). It is preferable that a substrate for a surface acoustic wave element have a TCD that is as close to zero as possible and a K.sup.2 that is as large as possible.
As stated previously, a conventional substrate for a surface acoustic wave element is typically produced by cutting and polishing a piezoelectric single crystal of lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), lithium tetraborate (Li.sub.2 B4O.sub.7), crystallized quartz, or the like, by a pertinent cut face. In the case of LiNbO.sub.3 (for example, 128.degree. Y cut-X propagation) K.sup.2 is as large as 5.5% but TCD is as large as 74 ppm/.degree. C. Accordingly, temperature changes cause the frequency to drift, and the substrate cannot be used in a filter requiring a narrow band characteristic, an oscillator requiring high stabilization, or the like.
Further, in the case of Li.sub.2 B.sub.4 O.sub.7 (for example, 45.degree. X cut-Z propagation), although K.sup.2 is 1% and TCD is 0 ppm/.degree. C., the substrate is dissolved in water and it bears a deliquescence property. Accordingly, processing is difficult and reliability is diminished.
Although ST cut crystallized quartz has a TCD of zero, K.sup.2 is as small as 0.1%. Therefore, a filter having a wide band cannot be obtained.
Due to the demand for a high performance surface acoustic wave element that outperform conventional devices, a chemically stable substrate material with a TCD near to zero and a large K.sup.2 is needed.